Because of significant advantages such as ease of information sharing and management, saving of physical space, and less susceptible to data loss, storing documents electronically is becoming a common practice. Once images of documents are stored in a computer system, there exist quite a number of technologies, mostly in the form of software, to properly mark, index, store, bundle, and search these images. The demand for scanning paper-type documents into electronic files or documents has therefore increased significantly in the recent years. Two types of scanning devices currently are available commercially for converting paper-type documents into electronic documents. The first type is a so-called glass top flatbed scanner that is capable of scanning at speed of 0.033 to 0.143 pages per second. The second type is a sheet-feed scanner that is capable of scanning at speed up to 3 pages per second. Sheet-feed scanner can efficiently scan documents with uniform physical shape. Time saving becomes especially significant when the page number of a document to be scanned is large. Because not all documents can be fed through the slot of the sheet-feed scanners, a flatbed device is essential for offices and individuals. For instance, in account receivable office of hospitals, clinics, and various companies, large volumes of checks and payment explanation sheets arrive through mail daily. Although these paper documents are good candidates for electronic storage, because they vary significantly in sizes and shapes, often folded and stapled in some way, there is a lack of efficient means for scanning these documents into a computer system due to the speed limitation. Today these paper documents may still be sorted, marked, bundled, and searched manually.
Current commercial flatbed scanner is made up of a scan head with light source, mirrors, focus lens and optical sensor. All parts in scan head move together during scanning. The sensor receives optical signals scanned from a document and converts the optical signals into electric signals, which are then processed into images of the document.
Factors limiting the scan speed of the flatbed devices are, among other things, the moving speed of the scan head, line scan rate of optical sensor, data transferring speed, and image processing speed. The image processing speed can be achieved to far surpass the moving speed of the scan head. Data transferring speed depends on the choices of protocols. The most common universal serial bus (hereinafter “USB”) port can transfer data at 1.5 MBps (megabyte per second). It only takes 0.2 second to transfer a 300 KB image file. Other protocols such as small computer system interface (hereinafter “SCSI”) are orders of magnitude faster than USB ports. The new generation of highly sensitive optical sensors can scan up to 46,000 lines, for example, DALSA IT-P 1-2048 (DALSA Corp., Waterloo, Ontario, Canada) per second. If each page has 4,000 lines, the sensor is able to finish scanning in less than 0.1 second. The fact that current sheet-feed scanners and copiers can scan up to 190 pages per minute (ppm) has proved that none of the image processing speed, data transferring speed, and sensor line rate is a speed bottleneck.
The speed bottleneck for the flatbed devices is the slow moving scan head. More specifically, it is not that the stepping motor cannot drive the scan head fast enough, it is the back and forth movement of the scan head and the start-stop action that limit the moving speed of the scan head. Therefore, despite orders of magnitude increase in microprocessor speed and memory density in recent years, the increase in flatbed scan speed of the flatbed scanners has been incremental.
Comparing to the 0.033 to 0.143 pages per second scanning time required by a glass top flatbed scanner, video camera employing area sensors could capture image of a document instantly. One example of such an image scanning system is disclosed in U.S. Pat. No. 5,511,148 entitled “Interactive Copying System”. Another example of such an image scanning system is disclosed in U.S. Pat. No. 6,493,469. It is understood that U.S. Pat. No. 6,747,764 also discloses a “camera box” like device in which an area sensor faces up to capture an image of a document that is faced down. The document is placed on top of a transparent platform. However, video camera employing area sensors usually do not have enough resolutions to replace an ordinary office scanner. Similar to the glass-top scanning devices, the flashing light emitting out of the scan area during scanning may discomfort and harm users and the device has limited scan area while the height of the device is high, due to the need for keeping sufficient distance between the to be scanned document and the area sensor.
Cameras employing line sensors, called line scan cameras, may produce higher image resolutions than one produced by video cameras employing area sensors. However, using line scan cameras for document scanning is inconvenient, for example, the document to be scanned usually needs to be faced up. Otherwise, a bulky scanning device needs to be constructed so as to place the line scan camera there below the document to be scanned. In addition, the line scan camera devices require strong scan lighting, which is uncomfortable for human eyes during frequent image capturing.
Based on the principle of the line scan camera, a scanner with a rotary mirror may work as a glass-top (or flatbed) scanner. But there are issues to be resolved before rotary mirror scanners become a popular product for routine use. One critic issue is that a certain distance is required between line sensors and the scan area. FIGS. 1 and 2 show a scanner utilizing a line sensor and a rotary mirror. The document to be scanned is placed on the surface of scan area 1 and faced down. A rotary mirror 2 reflects the imaging light of the original document to a condenser lens 3 and then to a line sensor 4. The rotary mirror is rotating around axis 5. This type of image scanner can scan original document at very high speed. However, this type of scanner has bulky construction and several other problems. As shown in FIG. 2, a viewing angle α0 is defined as the angle between the image path of the original document and the surface plane 1 of the original document at the far end of the scan area. The smaller the angle α0, the lower the resolution of the image taken from the original document near the far edge of the scan area, even after the distortion of the scanned image is eliminated. To maintain a certain resolution level, angle α0 must be greater than a certain threshold value at all times during scanning. Therefore, for scanning originals with certain size L0, the height H0 of the scanner cannot be made too small.
There are various attempts to shorten the distance between a document and rotary mirror and condenser lens. For examples, U.S. Pat. No. 6,396,648 uses a fish-eye lens and U.S. Pat. No. 6,324,014 uses a set of lenses. However, the shortening of the distance achieved by using different lenses is limited and further complicated by side effects of increasing distortion of image captured.
U.S. Pat. No. 6,493,469 takes two partial images of a document through two area sensor cameras. Each partial image has relatively small distortion and good resolution. The two images are combined to form a complete image of the original document. This design has several problems. Image capturing device such as a digital camera employing pricy area sensors that generally do not have sufficient resolution to replace the ordinary office scanners. The design also requires the document be placed facing up and camera facing down, occupies a relatively large space, is therefore not as convenient as flatbed scanners for frequent scanning. The proposed method to combine the two partial images is based on the captured images, which are unreliable in achieving a high-quality combined image.
U.S. Pat. No. 5,909,521 also discloses multiple partial images approach to obtain a complete image of a document. The image processing, however, is quite complex and the quality of alignments of partial images varies from scan to scan. As a result, the approach does not provide means for quickly and reliably combining partial images into one.
Another issue in designing rotary mirror scanner is to accurately time the coordination between image processing and angular position of the rotary mirror. Various methods have been proposed, for example, U.S. Pat. No. 6,088,167 discloses a method to use a dedicated light sensor, other than the image capturing optical sensor, to regularly capture the light beam when the light beam is in certain position so as to achieve the measurement of scanning position.
U.S. Pat. No. 5,757,518 proposes several methods to time the rotation of the rotary mirror. The first method is to count main scanning cycles, which requires no extra hardware for timing the scanning. However, timing main scanning cycles is difficult to implement if multiple partial images of the same original document need to be taken before they are combined into one complete image. Additionally, cycle counting errors may be accumulated. The second method is to measure angular displacement of reflecting mirrors. This method has the obvious drawback that extra components are needed. The third method of using an optical path length finder is also not desirable for the same reason.
U.S. Pat. No. 5,253,085 uses a synchronous sensor to detect the angular position of rotary mirror. Extra sensor and hardware mechanism are used to detect the angular position in U.S. Pat. No. 5,973,798.
The third issue in the design of a rotary mirror scanner is the correction of uneven shading of the image caused by uneven lighting exposure. It is known in the art of scanner that a standard white reference is needed for obtaining reference light intensity in order to lookup or compute shading data. There are numerous methods developed for shading correction, such as in U.S. Pat. No. 6,061,102, U.S. Pat. No. 5,724,456, U.S. Pat. No. 6,546,197, U.S. Pat. No. 6,195,469, U.S. Pat. No. 5,457,547, and U.S. Pat. Pub. No. U.S. 2003/0,142,367. However, in rotary mirror scanner design, the shading unevenness across the scan area would be greater than that in normal flatbed scanners. A much larger “standard white reference” area is therefore needed.
The fourth issue in the design of an image scanner is the elimination of distortions in the raw image initially taken by sensors. The captured raw images need to be processed to obtain undistorted image of the scanned documents. There are also numerous methods developed for distortion elimination, such as in U.S. Pat. No. 6,233,014, U.S. Pat. No. 5,253,085, and U.S. Pat. No. 6,219,446.
Still another issue is the strong light emitting out of the scan area during scanning. For very fast scanning, one method is to use a strong light to cover the entire scan area. This can be very uncomfortable for the eyes of a human operator. Another method is to use a strong and narrow light beam to scan the scan area following the imaging scan line. The second method minimizes the amount of illuminating light emitting out of the scan area during scanning. The light beam however, needs to scan the scan area in perfect synchronization with the imaging scanning. No solution so far has been provided to implement the second method.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.